The present invention relates to a control system in a vehicular automatic transmission for modulating the oil pressure of a hydraulic servomechism to engage or release frictional engagement means such as clutches.
Generally speaking, an automatic transmission is equipped with a gear transmission mechanism, a plurality of frictional engagement means and hydraulic control means. The automatic transmission is constructed to set a plurality of gear changes by engaging predetermined one of the frictional engagement means by the oil pressure supplied from the hydraulic control means.
The engagement and release of those frictional engagement means are accomplished to supply and drain the oil pressure to and from a hydraulic servomechanism which is composed main of a cylinder, a piston and a return spring. The engagement of the frictional engagement means is generally effected to reduce a large speed difference between two frictional members into a state of "0" relative speed. In order to smoothed this reduction, there is provided an accumulator. Specifically, a rise in the oil pressure at the hydraulic servomechanism is suppressed, while the oil is being accumulated in the accumulator, the suppression being according to the characteristics of the accumulator.
On the contrary, the release of the frictional engagement means is accomplished to disengage the two frictional members in the state of "0" relative speed by draining the hydraulic servomechanism of its oil. In this case, the draining rate of the oil is not a considerable problem. In the prior art, therefore, no pressure modulating mechanism is elaborately provided for the oil pressure to be drained from the hydraulic servomechanism.
As, however, there has been developed in recent years an automatic transmission which is capable of establishing multiple speed changes of five or six speeds, there appears an automatic transmission which is required to control not only the engaging rate but also the releasing rate of one frictional engagement means.
In this automatic transmission, two pressure modulating mechanisms, i.e., an accumulator for modulating the oil pressure to be supplied and a pressure modulating valve for modulating the drain oil pressure are provided for one hydraulic servomechanism.
Despite of this structure, however, the mere connection of the two pressure modulating mechanisms to one hydraulic servomechanism would exert influences on the oil pressure modulated by one modulating mechanism upon the other pressure modulating mechanism through the hydraulic servomechanism. As a result, the two pressure modulating mechanisms would then interfere each other to raise a trouble that the pressure modulating characteristics, as expected, could not be achieved.